Using current photolithography practices, a number of semiconductor devices can be formed on the same silicon substrate. One technique for isolating these different devices from one another involves the use of a shallow trench between two devices, or active areas, that is filled with an electrically-insulative material. Known as shallow trench isolation, a trench is formed that extends from a top material layer on a wafer to a buried oxide layer, for example, and the trench is then filled with an electrically-insulative material, such as oxide. In particular, chemical vapor deposition (CVD) is used to cover the entire wafer with the oxide material and then planarized.
This method of filling the trench with oxide introduces a number of problems. First, the oxide, typically silicon dioxide, must be planarized across the entire wafer to a level that coincides with the top of the trench. Through a planarizing process, such as chemical mechanical polishing (CMP), all the oxide must be completely removed from the active areas without over polishing either the active areas or the trenches. As wafer sizes have increased, uniform polishing over the entire wafer is difficult to accomplish and, as a result, some areas of the wafer have too much of the oxide removed while other areas have too little removed. Especially as wafer sizes have increased to 300 mm, “dishing”, or over polishing of the oxide is a common occurrence.
Additionally, CVD deposition of oxide results in growth from the bottom and sides of the trench. Thus, three growing fronts exist within the trench as the oxide is being formed. When two growing fronts meet, a seam is formed that behaves differently during wet etching, such as with buffered hydrofluoric acid (BHF) or diluted hydrofluoric acid (DHF). When etched with a wet etching solution, the seams etch at a faster rate than the other portions of silicon dioxide. As a result, trenches, or cavities, are formed in the silicon dioxide along the seams. During later fabrication steps that deposit material on the wafer, these cavities can collect the deposited material resulting in unintended consequences. For example, deposition of polysilicon followed by a polysilicon etch step will result in polysilicon unintentionally remaining in some of the cavities along the seams in the silicon dioxide. Under these circumstances, if two gate conductors cross a common seam, then an electrical short could develop between the conductors.
FIG. 1 illustrates a silicon-on-insulator (SOI) wafer 100 with shallow tench isolation regions formed using the conventional methods just described. In this figure, a silicon substrate 102 supports a buried oxide layer 104 and a SOI layer 106. In four active areas 120, 122, 124, 126, a pad oxide layer 108 and pad nitride layer 110 cover the SOl layer 106. Three trenches are formed between the active areas 120, 122, 124, 126 and are filled with an electrically-insulative oxide such as silicon dioxide 112. Because the silicon dioxide 112 is thermally grown using a CVD process, the silicon dioxide 112 in each trench includes seams 114 where growth fronts met when the silicon dioxide 112 was being formed. Furthermore, FIG. 1 depicts the over and under polishing that occurs when a thick layer of silicon dioxide 112 must be planarized over the entire surface of the wafer 100. For example, the right-side of the wafer 100 shows that the planarization step removed silicon dioxide 112 from the trench while the left-side of the wafer 100 shows that some silicon dioxide 112 still remains on the pad nitride layer 110.
Accordingly, there remains a need within the field of semiconductor fabrication for a shallow trench isolation technique that minimizes the mechanical polishing needed to planarize the oxide layer and that utilizes an oxide layer that has a uniform etch rate.